Military chiefs from the United States and its European and Middle Eastern allies met in Jordan for what could be a council of war, should they decide to punish Assad, who has denied using chemical weapons and blamed rebels for staging such attacks.
Many hundreds of people died in Damascus suburbs in what appears to have been the worst chemical weapons attack since Iraqi leader Saddam Hussein fatally gassed thousands of Kurds in 1988.
U.N. investigators crossed the front line from the centre of the capital, which remains under Assad's control, to inspect the Mouadamiya suburb, one of at least four neighbourhoods hit by the poison gas before dawn last Wednesday.
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The United Nations said one vehicle in its convoy was crippled by gunshots fired by "unidentified snipers." The team continued on after turning back for a replacement car. Syrian state television blamed rebel "terrorists" for the shooting. The opposition blamed pro-Assad militiamen.
"I am with the team now," a doctor who uses the name Abu Karam told Reuters by telephone from Mouadamiya. "We are in the Rawda mosque and they are meeting with the wounded. Our medics and the inspectors are talking to the patients and taking samples from the victims now."
Wassim al-Ahmad, an opposition activist, said members of the Free Syrian Army umbrella rebel organisation and the opposition's Mouadamiya Local Council were accompanying the inspectors on their tour of the suburb.
"The inspectors are now examining victims being treated at a makeshift hospital in Mouadamiya and are taking blood samples from them," Ahmad said.
Video filmed at the site showed inspectors in black and blue body armour and blue U.N. helmets walking through a street as curious onlookers came up to watch.
They shook hands with men who appeared to be rebels wearing camouflage vests, and were accompanied by doctors and residents.
The group descended into the basement of a building where they were told injured survivors were being treated to protect them from more shelling. Another video showed an inspector interviewing a patient and taking notes.
Activists say at least 80 people were killed in Mouadamiya when the district was hit with poison gas. Hundreds of people also were killed in three other rebel-held districts - Irbin, Ain Tarma and Jobar.
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An opposition activist said a large crowd of people gathered to air their grievances to the U.N. inspectors, who planned to take samples from corpses.
The inspectors later returned to their hotel and, within an hour, residents reported the shelling of Mouadamiya had resumed.
The decision to proceed with the mission despite coming under attack thwarted an apparent attempt to halt the inspectors' work before it began.
"The first vehicle of the Chemical Weapons Investigation Team was deliberately shot at multiple times by unidentified snipers in the buffer zone area," the United Nations said in a statement. "It has to be stressed again that all sides need to extend their cooperation so that the team can safely carry out their important work."
The inspectors had been stuck in a downtown hotel since the attack, waiting five days for government permission to visit the scene a few miles away. They had arrived three days before the incident, with a mandate to investigate earlier reports of more limited chemical weapons use.